Java Strings

A String is a sequence of characters, like a name or a sentence. It is an object (not a primitive) and it is used in almost every program.

Create a string

public class Main {
    public static void main(String[] args) {
        String a = "Hello";                 // string literal
        String b = new String("Hello");     // a new object
        String empty = "";
        System.out.println(a + " " + b + " [" + empty + "]");
        System.out.println(a.length());
    }
}

Join strings (concatenation)

public class Join {
    public static void main(String[] args) {
        String first = "Riya";
        String last = "Sharma";
        System.out.println(first + " " + last);
        System.out.println(first.concat(" ").concat(last));
        System.out.println("Age: " + 25 + 1);       // text + number: joined as text
        System.out.println(25 + 1 + " is the age"); // number + number first
    }
}

Comparing strings: equals, not ==

== checks if two variables point to the same object. equals() checks if the text is the same. Always use equals() for text.

public class Compare {
    public static void main(String[] args) {
        String a = "java";
        String b = "java";
        String c = new String("java");

        System.out.println(a == b);          // true: same pooled object
        System.out.println(a == c);          // false: different object
        System.out.println(a.equals(c));     // true: same text
        System.out.println("JAVA".equalsIgnoreCase(a));  // true
        System.out.println("apple".compareTo("banana")); // negative: apple comes first
    }
}

Java keeps all string literals in a special area called the string pool, so two equal literals are the same object. A string made with new is a separate object.

Strings are immutable

You can not change a string after it is created. Methods like toUpperCase() give a new string:

public class Immutable {
    public static void main(String[] args) {
        String s = "java";
        s.toUpperCase();                 // result is thrown away
        System.out.println(s);
        s = s.toUpperCase();             // keep the new string
        System.out.println(s);
    }
}

Special characters

Escape	Meaning
\n	New line
\t	Tab
\”	Double quote
\’	Single quote
\\	Backslash

public class Escape {
    public static void main(String[] args) {
        System.out.println("Line 1\nLine 2");
        System.out.println("Name:\tRiya");
        System.out.println("She said \"Hello\"");
        System.out.println("C:\\temp");
    }
}

Null and empty

public class Null {
    public static void main(String[] args) {
        String empty = "";
        String nothing = null;
        System.out.println(empty.isEmpty());      // true
        System.out.println(nothing == null);      // true
        System.out.println(" ".isBlank());        // true: only spaces
    }
}

Calling a method on null, like nothing.length(), throws a NullPointerException.

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